Fix sljit platform detection with xlc on AIX (#3329)
Fix sljit platform detection with xlc on AIX (#3329)

The XL C compiler's _ARCH_* macros are based on the -qarch setting and
    do not necessarily imply the object mode. In particular the _ARCH_PPC64
    macro only indicates that the compiler is targeted to run on a Power
    processor with 64-bit support. The __64BIT__ macro is needed to detect
    that the compiler is actually in 64bit mode.

    This reapplies commit f417f3333b2212f19d23554ccb7802d3be3227b3
    Author: Jeremy Devenport <jdevenport@bloomberg.net>
bloomberg/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde